Huntington's Disease (HD) is a familial fatal neurodegenerative disease caused by expression of expanded polyglutamine tracts in the (Htt) protein. Currently there are no effective cures or treatments for HD. Current drug screening efforts to discover potential therapeutics for HD include the use of in vitro aggregation assays, cell-based, and model organism based Htt toxicity assays. The precise molecular details of HD pathology are not fully understood, however it is clear that some aspects of expanded Htt toxicity are driven by aberrant protein-protein interactions involving the mutant Htt and other cellular proteins. Direct protein-protein interaction of mutant Htt with the transcription factor Spl, and with the transcriptional co-activator CBP have been implicated as key pathologic mechanisms in HD. To date, about 40 protein interaction partners for Htt have been described, and it is likely that some of these interactions will also have causative roles in HD pathology. Given the prominent role for protein interactions in HD pathology, it is likely that drugs which could specifically disrupt pathogenic Htt protein interactions could be of therapeutic benefit. The primary goal of this proposal is to develop cell-based and in vitro protein interaction assays suitable for high throughput screening (HTS). These will be used to discover compounds able to disrupt Htt protein-protein interactions. One assay will be a yeast-based "reverse two hybrid" assay employing a counter-selectable reporter gene. This yeast assay is configured such that Htt-protein interactions are lethal, and can thus be used to screen for compounds that suppress lethality. A complementary second assay will be an in vitro fluourescence resonance energy transfer (FRET)-based protein interaction assay which will be used to screen for compounds able to inhibit protein interactions in simple binary systems. These assays will be used in parallel to screen for compounds able to inhibit protein interaction between Htt and 10 different huntingtin binding partners. Two of these partners will be CBP and Spl, and eight others will be chosen from an on-going protein interaction discovery program. After assay development and validation, initial screens will be performed with a small chemical library (NINDS custom collection). These screening tools and hits will be further developed internally, as well as through collaborations with academic and foundation-based HD therapeutic discovery efforts.